1. Field of the Invention
The present invention relates to a surface shape measurement apparatus for measuring a shape of a surface to be measured.
2. Description of the Related Art
In recent years, optical systems mounted in, for example, a camera, copying machine, telescope, and exposure apparatus popularly use aspherical optical elements such as aspherical lenses. Furthermore, a surface to be measured includes, for example, that having a free-form curve shape, that having significant undulation, and that having a steep slope. Therefore, a surface shape measurement apparatus is required to have a function of measuring various surfaces to be detected described above. Japanese Patent Laid-Open No. 2002-116010 discloses a three-dimensional shape measurement apparatus. This measurement apparatus measures a shape of an object surface to be measured by using light (return light), which is obtained by irradiating the object surface to be measured with a spherical wave via a small aperture, reflecting the spherical wave by the object surface to be measured, and returning the reflected wave via the small aperture. More specifically, this measurement apparatus detects a tilt angle of the return light with respect to an optical axis using a tilt angle measuring unit, and detects, using a displacement amount detecting unit, a displacement amount of a distance between the reflected position of the light on the object surface to be measured and the small aperture upon scanning the object surface to be measured. Then, this measurement apparatus calculates the shape of the object surface to be measured based on the detected tilt angle and displacement amount.
In a surface shape measurement apparatus, which measures a shape by scanning a measurement head (probe), it is desirable that the measurement head is compact and lightweight. The compact measurement head contributes to a reduction of a space occupied by the measurement head in the measurement apparatus to broaden a scanning range. The lightweight measurement head contributes to stable measurement precision by eliminating a small deformation of a structure due to partial weighting on the structure of the measurement apparatus upon movement of the measurement head. Since the apparatus described in Japanese Patent Laid-Open No. 2002-116010 is required to detect both the tilt angle and displacement amount, the measurement head gets bulky and heavy accordingly.